Inductor and electric power supply using it

ABSTRACT

An inductor embedded in a printed wiring board includes a conductor extending in the thickness direction of a printed circuit board and a magnetic body that is in contact with the conductor with no gap therebetween. For example, the magnetic body is composed of ferrite having a cylindrical tubular shape. The conductor is composed of a copper film formed by plating on an inner peripheral surface of the cylindrical tubular ferrite. The inductor is inserted in the thickness direction of the printed wiring board.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims the benefit of priorityunder 35 U.S.C. §120 from U.S. Ser. No. 11/429,157, filed May 8, 2006,the entire contents are incorporated herein by references.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an inductor and an electronic powersupply using it. More specifically, the present invention relates to aninductor used in a smoothing circuit of a power supply circuit for largescale integrated circuits (LSIs) mounted on a printed wiring board, anda power supply circuit including the same.

2. Related Art

Recently, in semiconductor devices such as an LSI used in an electronicdevice, the driving voltage has been decreased to a very low value,about 1 volt, in order to achieve high performance and low electricpower consumption. In order to provide such an LSI load with a drivingelectric power, it is necessary to provide electric power obtained byrectifying an alternating current to a direct current and decreasing thevoltage in several stages. For such an application, A DC-DC converterwith excellent conversion efficiency is generally used. In this case,noises during output must be suppressed using a smoothing circuit.

The smoothing circuit mainly includes two types of elements, namely,inductors and capacitors for which surface-mount devices are mainlyused. The mounting of such surface-mount devices on a printed wiringboard requires a certain area for mounting.

According to an inductor disclosed in FIGS. 1 and 2 in Japanese PatentLaid-open Publication No. HEI 1-312885 “Circuit Board with an Inductorembedded therein”, (publication Date: Dec. 18, 1989), a cylindricalferrite body 20 is fitted in a through-hole 18, and a conductor 24 isinserted into a through-hole 22 of the cylindrical ferrite body 20.

However, microscopically, unlike an “inductor including a conductor anda magnetic body that is in contact with the conductor with no gaptherebetween” described below, this structure includes an gap betweenthe conductor and the magnetic body, and thus a high inductance cannotbe obtained.

SUMMARY OF THE INVENTION

In order to reduce the size of an electronic device and to achievehigh-density mounting thereof, surface-mount devices constituting thesmoothing circuit are disadvantageous in that the mounting area on aprinted wring board is relatively large and the cost of the devices ishigh.

Accordingly, it is desirable to develop devices (inductors andcapacitors) whose mounting area is relatively small.

Accordingly, it is an object of the present invention to provide a novelinductor and a method of producing the same.

Furthermore, it is another object of the present invention to provide apower supply circuit including the novel inductor.

The inductor according to this invention constituting a part of a powersupply circuit comprises a magnetic body having a through-hole and aconductor formed on a surface of the through-hole. The conductor of theinductor may be composed of copper. The conductor may have asubstantially cylindrical shape. The conductor of the inductor also mayhave a substantially hollow cylindrical shape. The magnetic body of theconductor may be of a shape such that the magnetic body substantiallysurrounds the conductor. The magnetic body of the conductor may becomposed of ferrite. The magnetic body of the conductor may be composedof a composite material containing a magnetic material and a nonmagneticmaterial. The magnetic body of the conductor may be composed of acomposite material containing a magnetic powder and a resin. Themagnetic body of the conductor may be composed of a composite materialcontaining a carbonyl iron powder and a resin. The inductor may furthercomprise a dielectric material that may be of a shape such that thedielectric material substantially surrounds the magnetic body.

An inductor of this invention embedded in a printed wiring board,comprises a magnetic body that extends in the thickness direction of theboard and that has a through-hole and a conductor formed on an innersurface of the through-hole. The conductor of the inductor may becomposed of copper. The conductor of the inductor may a substantiallycylindrical shape. The conductor of the inductor may have asubstantially hollow cylindrical shape. The magnetic body of theinductor may surround the side face of the conductor. The magnetic bodyof the inductor may be composed of ferrite. The magnetic body of theinductor may be composed of a composite material containing a magneticmaterial and a nonmagnetic material. The magnetic body of the inductormay be composed of a composite material containing a magnetic powder anda resin. The magnetic body of the inductor may be composed of acomposite material containing a carbonyl iron powder and a resin. Theinductor embedded in the board may further comprise a dielectricmaterial that may surround the side face of the magnetic body. Thedielectric material of the inductor may be composed of an under-fillresin material having low thermal expansion characteristics.

An electronic device of this invention comprises a board and a powersupply circuit mounted on the board that supplies a semiconductor devicewith power; wherein the power supply circuit includes at least aninductor formed in the thickness direction of the board. The powersupply circuit of the electronic device may include a thin-filmcapacitor formed in the direction parallel to one principal surface ofthe board; an inductor formed in the thickness direction of the boardand a power supply IC device mounted on another principal surface of theboard. Further, the power supply circuit of the electronic device mayinclude a thin-film capacitor formed in the direction parallel to oneprincipal surface of the board; an inductor formed in the thicknessdirection of the substrate and a power supply IC device mounted onanother principal surface of the board; the thin-film capacitor, theinductor embedded in the board, and the power supply IC device aredisposed close to the semiconductor device to connect between the powersupply circuit and the semiconductor device with a short conductivecircuit. The inductor of the electronic device may include a magneticbody that extends in the thickness direction of the board and that has athrough-hole, and a conductor formed on the surface of the through-hole.A plurality of sets of the power supply circuits of the electronicdevice may be provided in the board.

A method of producing an inductor of this invention comprises the stepsof providing a magnetic body extending along a longitudinal axis,forming a though-hole in the axial direction of the magnetic body, andperforming plating with a metal on an inner surface of the though-holeto stick the metal on the magnetic body. The metal plating of the methodfor producing an inductor may be copper plating. The magnetic body ofthe method for producing an inductor may be composed of ferrite. Themagnetic body of the method for producing an inductor may be composed ofa composite material containing a magnetic material and a nonmagneticmaterial.

A method of embedding an inductor in a board of this invention comprisesthe steps of preparing an inductor that is made by preparing alongitudinally extending magnetic body and by performing metal platingon the inner surface of a through-hole formed in the axial direction ofthe magnetic body; forming a through-hole through the board; insertingthe inductor into the through-hole; and filling a space between theinductor and the board with a resin to fix the inductor to thesubstrate. The method of embedding an inductor in a board may furthercomprise the step of filling the through-hole of the inductor with aresin. In the method of embedding an inductor in a board, both ends ofthe through-hole of the inductor may be covered with a metal. The methodof embedding an inductor in a board may further comprise the steps of,after the inductor is fixed to the substrate, performing plating withcopper on the surfaces of the board and on the inner surface of thethrough-hole of the inductor and patterning it.

Also, a method of embedding an inductor in a board of this invention,comprises the steps of preparing a cylindrical magnetic body, forming athrough-hole through the board, inserting the magnetic body into thethrough-hole, filling a space between the inductor and the board with aresin to fix the inductor to the board, performing plating with copperon the surface of the board and on the inner surface of the through-holeof the magnetic body and patterning it. The method of embedding aninductor in a board may further comprise the step of filling up thethrough-hole of the inductor with a resin.

According to the present invention, a novel inductor and a method ofproducing the same can be provided.

Further, according to the present invention, a power supply circuitincluding the novel inductor can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing the structure of a printedcircuit board, constituting an electronic device, on which a powersupply circuit is mounted;

FIG. 2 is a circuit diagram showing the fundamental structure of thepower supply circuit used in the electronic device shown in FIG. 1;

FIGS. 3A to 3D are waveform charts of the voltage or the currentillustrating the operation of the power supply circuit shown in FIG. 2,wherein the abscissa of each chart is a time base t;

FIG. 4A is a cross-sectional view showing a state in which an inductoraccording to an embodiment of the present invention is embedded in apackage;

FIG. 4B is a cross-sectional view showing a state in which an inductoraccording to another embodiment of the present invention is embedded ina package;

FIG. 4C is a cross-sectional view showing a state in which an inductoraccording to another embodiment of the present invention is embedded ina package;

FIG. 5A is a view specifying the dimensions of the inductors shown inFIGS. 4A to 4C;

FIG. 5B is a table showing a comparison between the inductances ofinductors having a ferrite core and those of inductors having a FR-4core;

FIG. 5C is a graph showing a comparison of the inductances when thecurrent is increased from 0.01 to 10 A;

FIG. 5D is a B-T characteristic diagram showing a comparison between theinductances of an inductor having a composite material core and those ofan inductor having a ferrite core;

FIG. 6 includes views showing a method of producing an inductor;

FIG. 7A includes views showing a method of embedding a produced inductorin a printed wiring board, according to an embodiment of the presentinvention;

FIG. 7B includes views showing a method of embedding a produced inductorin a printed wiring board, according to another embodiment of thepresent invention;

FIG. 7C includes views showing a method of embedding a produced inductorin a printed circuit board, according to another embodiment of thepresent invention;

FIGS. 8A to 8G include views showing a method of manufacturing a printedwiring board used for the electronic device shown in FIG. 1; and

FIG. 9 is a view showing an existing printed circuit board constitutingelectronic device, on which a power supply circuit is mounted.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of an inductor according to the present invention and apower supply circuit including the same will now be described in detailwith reference to the accompanying drawings. It should be understoodthat these embodiments are illustrative and do not limit the scope ofthe present invention. In the figures, the same reference numeral isassigned to the same element, and the repeated description of thiselement is omitted.

[An Electronic Device on which a Power Supply Circuit is Mounted]

(Structure)

FIG. 1 is a cross-sectional view showing the structure of an electronicdevice 1 made by a printed circuit board equipped with a power supplycircuit. The electronic device 1 includes a package (PK) 2 and amotherboard (MB) 4 mounted the package 2 thereon, and the both areelectrically connected to each other with, for example, a pin junction16. The motherboard 4 is composed of a suitable printed circuit board;and a DC voltage V_(in) having an appropriate magnitude is supplied fromthe outside to a conductor circuit 14 of the motherboard 4 to send thevoltage V_(in) to the package 2 through the pin junction 16.

The package 2 is composed of a suitable printed circuit board, and, forexample, the package 2 includes a core substrate 13, lower insulatinglayers 15 u and 15 d provided on the top and bottom surfaces of the coresubstrate 13, respectively, upper insulating layers 25 u and 25 dprovided on the surfaces of the lower insulating layers 15 u and 15 d,respectively, and, if required, solder resist layers 35 u and 35 dcovered on the surfaces of the upper insulating layers 25 u and 25 d,respectively. Conductor circuits 52 u and 52 d are formed on the top andbottom surfaces of the core substrate 13, respectively, and athrough-hole conductor 28 is formed to connect the conductor circuits 52u and 52 d. Conductor circuits 54 u and 54 d are formed on the lowerinsulating layers 15 u and 15 d, respectively, and via-hole conductors53 u and 53 d are formed in the lower insulating layers 15 u and 15 d,respectively. Similarly, conductor circuits 56 u and 56 d are formed onthe upper insulating layers 25 u and 25 d, respectively, and via-holeconductors 55 u and 55 d are formed in the upper insulating layers 25 uand 25 d, respectively. Preferably, the core substrate 13 is formed by aplated through-hole manufacturing process, and the lower insulatinglayers 15 u,15 d and the upper insulating layers 25 u,25 d are formed bya build-up manufacturing process.

The package 2 further includes inductors (L) 10 embedded in the boardand a thin-film capacitor (C) 8 formed with conductor circuits 52 u-1and 54 u-1. Furthermore, a semiconductor device (microprocessor unit(MPU)) 6 is mounted on the top surface of the package 2, and a powersupply IC (PW IC) 12 is mounted on the reverse face of the package 2preferably at the position corresponding to MPU 6 mounting position.

The thin-film type capacitor (C) 8 is formed by interposing a dielectricmaterial 8 between the core substrate conductor circuit 52 u-1 and thelower-layer conductor circuit 54 u-1, and is preferably provided in thevicinity of the MPU 6. The inductors (L) 10 embedded in the board andthe power supply IC (PW IC) 12 are described in detail below.

A circuit including all or any one of the thin-film capacitor (C) 8, theinductors (L) 10 embedded in the board, and the power supply IC (PW IC)12 constitutes the power supply circuit for supplying the MPU 6 withpower. The power is supplied from the power supply circuit to the MPU 6through the conductor circuits (including the through-hole conductorsand the via-hole conductors; hereinafter the same) formed in the package2. In this electronic device 1, the distance from the output of thepower supply circuit to the MPU 6, which is a load, is very short, forexample, 1 mm or less. Since the length of the conductor circuit usedfor supplying the power is very short, voltage variations due to aparasitic resistance or a parasitic inductance of conductor circuitwiring can be suppressed.

The inductors (L) 10 embedded in the board are formed in a part of thecore substrate 13 of the package 2. Alternatively, the inductors (L) 10may be formed in a part of or throughout the entire package 2. However,in the description below, for simplicity, the case where the inductors(L) 10 embedded in a part of the core substrate 13 is described as anexample.

Furthermore, the power supply to the package 2 is not limited to a setof power supply circuit (i.e., a set of the thin-film capacitor 8, theinductor 10 embedded in the board, and the power supply IC 12). When thepower demand for the MPU 6 etc. is high, plural sets of the power supplycircuits may be prepared and connected in parallel with each other sothat each power supply circuit shares the power demand. In this case,the number of the thin-film capacitors 8, the inductors 10 embedded inthe board, and the power supply ICs (PW ICs) 12 are determined accordingto the desired number of sets.

(Circuit and Operation)

FIG. 2 is a circuit diagram showing the fundamental structure of a powersupply circuit 3 embodied in the electronic device shown in FIG. 1. Thispower supply circuit 3 is a DC-DC converter that steps down the input DCvoltage. In this DC-DC converter, a power supply IC (PW IC) 12, aninductor (L) 10 embedded in a board, and an MPU 6, which is a load, areconnected to an input power V_(in) in series; furthermore, a thin-filmcapacitor (C) 8 is connected across the load MPU 6 in parallel. Thepower supply IC (PW IC) 12 includes a switching element (SW) 9 and adiode (D) 11, and the diode 11 is connected to the load MPU 6 inparallel to function as a flywheel diode. As a power supply for the loadMPU 6, the frequency of the switching element (SW) 9 is about 0.1 to 10MHz.

This DC-DC converter serves as a DC chopper circuit in the pre-stage tochange the average of the load voltage by changing the ratio of onecycle T to ON time t_(on) in the input voltage V_(in) with the powersupply IC (PW IC) 12; and in the post-stage smoothen the output voltagechanged in the pre-stage. The voltage generated at both ends of the loadis referred to as V_(out).

According to the fundamental operation of the DC-DC converter shown inFIG. 2, when the power supply IC (PW IC) 12 turns ON, current flows inthe inductor (L) 10, which is a choke coil, the thin-film capacitor (C)8, and the load MPU 6. At that time, electromagnetic energy isaccumulated in the inductor (L) 10 and the thin-film capacitor (C) 8.Then, when the power supply IC (PW IC) 12 turns OFF, the electromagneticenergy accumulated in the thin-film capacitor (C) 8 permits the currentto continue to flow to the load MPU 6. Similarly, the electromagneticenergy accumulated in the inductor (L) 10 continues to flow through thediode (D) 11, which is a flywheel diode. When the power supply IC (PWIC) 12 turns ON again, after the reverse recovery time of the diode (D)11 has elapsed, current flows in the inductor (L) 10, the thin-filmcapacitor (C) 8, and the load MPU 6, and electromagnetic energy is againaccumulated in the inductor (L) 10 and the thin-film capacitor (C) 8.

In this series of operations, the voltage V_(out) applied to the loadMPU 6 includes a ripple component, and the variations of the ripplecurrent and voltage are determined according to the magnitude of thereactance of the inductor (L) 10 and the thin-film capacitor (C) 8.Regarding the ripple component (current variation AIL), a designspecification value is determined in advance, and the ripple componentis suppressed by a smoothing circuit (i.e., a filter circuit composed ofthe inductor (L) 10 and the thin-film capacitor (C) 8) to reduce theripple component below the specified value. That is, even if the inputvoltage V_(in) has an interrupted waveform, the direct current flows inthe diode D 11, thereby providing a continuous waveform having thereduced ripple current. Furthermore, as the inductance of the inductor(L) 10 is increased, the direct current can be further smoothened.

On the other hand, this power supply circuit is used for the MPU 6 whosedriving voltage is about 1 volt and the voltage drop due to resistancemust be prevented as much as possible at such a low voltage. Therefore,the inductor (L) 10 must have a low resistance, a high inductance, and asmall size for the purpose of reducing the size of the electronic deviceand realizing a high-density mounting.

FIGS. 3A to 3D are waveform charts of the voltage or the currentillustrating the above operations. The abscissa of each chart is a timebase t. FIG. 3A shows an input DC voltage V_(in) in the case where theswitching element (SW) 9 is in the ON state, and FIG. 3B shows a currentIL that flows in the inductor (L) 10 in that case. The input DC voltageV_(in) and the direct current IL are constant as long as the switchingelement (SW) 9 is in the ON state.

As shown in FIG. 3C, when the switching element (SW) 9 turns ON for theperiod of time t_(on) with a cycle T, the output voltage caused by theinput voltage V_(in) results in rectangular waves in which the inputvoltage V_(in) is off and on. In this case, the average voltage of theinput voltage is small, V_(out)=(t_(on)/T)*V_(in).

Because of the presence of the inductor (L) 10 and the thin-filmcapacitor (C) 8, a voltage that is continuous, but includes a ripplecomponent, is applied to the load MPU 6. FIG. 3D shows a current thatflows in the inductor (L) 10. This current has an average of IL, avg.and a ripple current of ΔIL.

[Inductor]

(Structure)

FIG. 4A is a cross-sectional view showing a state in which an inductor10 is embedded in the package 2. The inductor (L) 10 preferably has alow resistance, a high inductance, and a small size. However, ingeneral, when the length of a conductor is increased, the inductance isincreased and the resistance is also increased. On the other hand, whena magnetic material is disposed in the vicinity of the conductor, theinductance is increased.

Therefore, the present inventors decided that the resistance is reducedby using metallic copper as the conductor for the inductor anddecreasing the length of the conductor. In addition, the inductance isincreased by disposing a magnetic material in the vicinity of theconductor. From this point of view, the inventors propose inductors 10embedded in a board shown in FIGS. 4A to 4C, as embodiments. Althoughthe following description relates to the case where the inductor 10 isformed in a part of the core substrate 13, the inductor 10 may be formedin any part of or through the entire of package 2.

An embedded inductor (L) 10 shown in FIG. 4A includes a conductor (con.)32 composed of through-hole copper and a core 30 composed of acylindrical ferrite body surrounding the conductor 32. The inside of thethrough-hole copper forms a hollow part 34. The hollow part 34 providedin the conductor 32 releases a stress caused by a difference in thethermal expansion between the conductor 32 and the core 30.

The inductor 10 is disposed in a through-hole 27 provided in the coresubstrate 13, and the periphery of the inductor 10 is covered with aresin 38. Via conductors 39 are formed so that the openings of both endfaces of the inductor 10 are covered, and the via conductors 39 areconnected to the conductor layers 52 u and 52 d.

FIG. 4B shows another embodiment of an embedded inductor (L) 10, where athrough-hole conductor 32 is electrically connected to conductorcircuits (i.e., a wiring patterns formed on the surfaces of the coresubstrate, a substantially sheet-shaped conductor circuits constitutinga power supply layer and/or a ground layer, or a through-hole lands forelectrically connecting a via-hole conductor to the upper layer). Thisthrough-hole land is not subjected to rewiring on a surface of the coresubstrate 13.

Referring to FIG. 4C, a filling agent 36 is filled in the through-holeconductor 32 shown in FIG. 4B, and lid-like conductors 39 covering thefilling agent 36 are provided. A via-hole conductor may be formed on thelid-like conductors 39. A material having a low elasticity is preferablyused for the filling agent 36 because a stress caused by a difference inthe thermal expansion between the magnetic body, the though-holeconductor, and the core substrate can be reduced.

(Performance)

FIGS. 5B to 5D show the performance of inductors 10 having a ferritecore shown in FIG. 5A. First, the case was examined that the thicknessof the ferrite core is varied. In this experiment, inductors having acore composed of an organic material (FR-4) (relative permeability=1)were used as comparative examples.

FIG. 5A is a view specifying the dimensions of the inductors (L) 10shown in FIGS. 4A to 4C. As shown in FIG. 5B, in the inductor samplesused in this experiment, the length of the conductor 32 and the lengthof the core 30 are the same, i.e., l_(con.)=l_(core)=1 mm. The radius ofthe conductor 32 is also the same, r_(con.)=1 mm. Under theseconditions, the core radius r_(core) was varied from 0.25 to 2 mm todetermine the variation in the inductance. The inductors of thecomparative examples that have the core composed of the organic material(FR-4) also have the same dimensions.

The inductances of the inductors 10 having the ferrite core weredetermined as follows: When the core radius r_(core) was 0.25 mm, theinductance was 5.70×10⁻⁸ H. When the core radius r_(core) was 0.50 mm,the inductance was 1.12×10⁻⁷ H. When the core radius r_(core) was 1 mm,the inductance was 1.74×10⁻⁷ H. When the core radius r_(core) was 2 mm,the inductance was 2.20×10⁻⁷ H. As these results, it was found that theinductance is dependent on the thickness of the ferrite core. When theseinductances are compared with those of the inductors having the corecomposed of the organic material FR-4, the inductance ratios are 40.5,79.6, 123.7, and 155.9. Since the conductor is in contact with themagnetic body without a gap therebetween, such high inductances can beachieved.

Next, other types of magnetic material were examined instead of theferrite magnetic material of the inductor shown in FIG. 5A. Here, acomposite material containing a magnetic material and a nonmagneticmaterial was proposed. In the experiment, an inductor having thecomposite material core, that contained a carbonyl iron powder servingas the magnetic material and a resin serving as the nonmagneticmaterial, was prepared and was compared with the inductor having theferrite core shown in FIG. 5A. Both inductors had a core radius r_(core)of 0.25 mm.

FIG. 5C is a graph showing a comparison of the inductances of the aboveinductors when the current was increased from 0.01 to 10 A. An inductor30-3 that does not include a core is shown as a comparative example.That is, the inductor 30-3 includes an air core (relative magneticpermeability=1) instead of the core composed of the FR-4 material(insulating material). In the inductor 30-1 having the ferrite core, theinductance is high in the current range of 0.01 to 0.1 A. When thecurrent was increased in the range of 1 to 10 A, the inductance issaturated and gradually decreased. As shown in FIG. 5B, when the currentfor measurement was 0.01 A and the core radius r_(core) was 0.25 mm, theinductance was 5.70×10⁻⁸ H. This inductance corresponds to theinductance value at the left end (at the current of 0.01 A) of the curve30-1 in FIG. 5C.

On the other hand, the inductance of the inductor 30-2 having thecomposite material core is lower than that of the inductor 30-1 havingthe ferrite core, however the inductance of the inductor 30-2 was aboutthree times in comparison with that of the inductor 30-3 having the aircore. Furthermore, the inductor 30-2 had a feature that even when thecurrent was further increased, the inductor 30-2 is maintained aspecific inductance.

FIG. 5D is a B-T characteristic diagram showing a comparison of theinductor 30-2 having the composite material core with the inductor 30-1having the ferrite core. When the magnetic field strength is increasedfrom 0 to 20,000 A/m, the inductor 30-1 having the ferrite core 3 wasrapidly magnetized because of its high relative permeability, butimmediately reached magnetic saturation. In contrast, the inductor 30-2having the composite material core was magnetized substantially inproportion to the magnetic field strength without causing magneticsaturation because of a relatively low relative permeability. It isbelieved that this property is the cause of the phenomenon shown in FIG.5C; wherein the inductor 30-1 having the ferrite core has a highinductance, but the inductance is saturated and gradually decreased asthe current increases. In contrast, the inductor 30-2 having thecomposite material core has a relatively low inductance, but itmaintains a certain inductance.

Since the inductor 10 of this embodiment is used in a power supplycircuit, a large amount of current may flow therein. When the currentvalue is relatively low, the inductor 30-1 having the ferrite core,which has a high inductance, is preferred. On the other hand, when thecurrent value is relatively high (for example, 0.1 A or higher, or 1 Aor higher), the inductor 30-2 having the composite material core, whichhas a relatively low inductance but maintains a certain inductance evenwhen the current is increased, is preferred. The final goal for thepresent inventors is to develop an inductor that has a high inductanceand that can maintain a certain inductance even when the current isincreased.

The inductor of this embodiment can be used as an inductor constitutinga part of a power supply circuit that controls high current in a highfrequency area (for example, in a switching power supply circuit, inorder to transform the AC power to the DC power, or in order to shield ahigh-frequency component from a direct current or a low-frequencyalternating current).

(Method of Producing an Inductor)

FIG. 6 includes views showing a method of producing an inductor usingferrite as the core material. When a composite material is used as thecore material, the method of producing the inductor is similar to thefollowing method.

A ferrite bulk material 30, which is a magnetic material, is prepared(step 1).

Subsequently, the ferrite 30 is formed into a cylindrical shape having athrough-hole 31 and is sintered (step 2). The dimensions are preferablycontrolled so that the sintered cylindrical ferrite 30 has a height of0.05 to 1.00 mm. The conditions for forming and sintering the ferritematerial are determined so that the sintered ferrite has a relativedensity of at least 95%, and preferably, at least 98%. In this case, theferrite 30 has a relative permeability of 100 to 150 and a saturationmagnetization of about 0.4 T (Tesla). Thus, the core part 30 is formed.Alternatively, this core part 30 may be formed by forming ferrite into acylindrical shape, sintering the formed ferrite, and then opening a holealong the longitudinal axis with a suitable tool such as a drill.

Subsequently, except for the through-hole 34, both end faces of thecylindrical ferrite 30 are covered with a resist film. A thin copperfilm is then formed on the surface of the ferrite (i.e., on the innerperipheral surface of the through-hole 31) by chemical copper plating(i.e., electroless copper plating). Subsequently, a copper film having athickness of about 20 μm is formed by copper pyrophosphate plating(i.e., electrolytic copper plating) to form a conductor 32 (step 3). Byforming the conductor 32 on the core part 30 by plating, the conductor32 is in contact with the core part 30 without a gap therebetween. Thedry resist film is then removed. In this case, if the through-holeconductor 32 projects from the core part 30, the projecting part isremoved by polishing.

Subsequently, as described in Japanese Patent Laid-open Publication No.2000-232078 (publication Date: Aug. 22, 2000), a plating headimpregnated with a plating solution is brought into contact with an endface of the core. Pads 32 t for measurement are then formed one by oneusing the through-hole conductor 32 as the cathode (step 4 a or 4 b).Ni/Au plating may be performed on the surface of each pad 32 t.

In the cross-sectional view, a hollow part 34, the conductor 32, and theferrite core 30 are concentrically disposed from the center (see X-Xcross-sectional view). Regarding the results shown in FIGS. 5B to 5D, amicroprobe of an impedance analyzer was brought into contact with thepads 32 t for measurement to perform the measurement.

In FIGS. 5B to 5D, the inductors 30-2 of the comparative examples wereprepared by the same method as that described above, i.e., with the samedimensions, with the same steps 1 to 4, and under the same conditions asthose in the above method, except that FR-4 (a flame-retardant epoxyresin) was used instead of ferrite.

(Method of Embedding Inductor in Substrate)

FIGS. 7A to 7C include views showing methods of embedding a preparedinductor 10 in a core substrate 13. As described above, the inductor 10may be embedded in a part of or throughout the entire package 2.

In a method of embedding an inductor shown in FIG. 7A, a through-hole 27for embedding an inductor is formed in a core substrate 13 with, forexample, a drill (step A-1). The inductor 10 prepared in steps 1 to 4 aor 1 to 4 b shown in FIG. 6 is inserted into the through-hole 27 (stepA-2). A resin 38 is packed around the inductor 10. This resin 38 ispreferably an under-fill resin having a low coefficient of thermalexpansion (CTE), which is used in a flip-chip mounting. Subsequently,openings 38 a are formed with a laser at the central parts of the resin38 disposed on both end faces (step A-3). Filled via conductors 39 areformed by electroless copper plating and electrolytic copper plating,and the via conductors 39 are connected to surface conductor layers 52 uand 52 d of the package 2 (step A-4). Preferably, a solder resist layermay be covered as an insulating layer.

In a method of embedding an inductor shown in FIG. 7B, a hole is formedin a core substrate 13 clad with a copper foil 40 (step B-1), aninductor 10 is inserted into the hole (step B-2), a resin 38 is packedaround the inductor (step B-3), electroless copper plating andelectrolytic copper plating are performed to form a copper film 42 (stepB-4 a), and the copper film 42 is patterned (step B-5 a). Alternatively,after step B-3, the opening of the inductor may be filled with a fillingresin 44 (step B-4 b), electroless copper plating and electrolyticcopper plating may be performed to form a copper film 42 (step B-5 b),and the copper film 42 may be patterned (step B-6 b). The core substrate13 may not include the copper foil 40.

A method of embedding an inductor shown in FIG. 7C is an example of amethod in which a core substrate 13 not including a copper foil 40 isused, a core 30 of an inductor 10 is inserted into the substrate, and aconductor 32 is then formed. A hole is formed in the core substrate 13(step C-1), cylindrical ferrite 30 formed in steps 1 and 2 shown in FIG.6 is inserted into the hole, and a resin 38 is packed around thecylindrical ferrite 30 (step C-2), electroless copper plating andelectrolytic copper plating are performed to form a copper film 42 (stepC-3), and the copper film 42 is patterned (step C-4 a). Alternatively,after step C-3, the opening of the inductor may be filled with a fillingresin 44 (step C-4 b), electroless copper plating and electrolyticcopper plating may be performed to form a copper film 46 (step C-5 b),and the copper films 42 and 46 may be patterned (step C-6 b). The coresubstrate 13 may include a copper foil 40.

In the methods of embedding an inductor shown in FIGS. 7A to 7C, a resinsubstrate or a ceramic substrate may be also used as the substrate.

(Method of Producing Printed Wiring Board)

A method of producing a printed circuit board used as the package 2 andthe motherboard 4 of the electronic device 1 shown in FIG. 1 will now bedescribed with reference to FIGS. 8A to 8G. Known processes ofmanufacturing a multilayer printed wiring board include a through-holeplating process and new processes. Examples of the new processes includea process combining a plating process and a build-up process, a processcombining a conductive paste application process and a build-up process,a process combining a build-up process and a transferring process, atransferring process, a process combining a columnar plating and abuild-up process, and a collective laminating process. Furthermore, thecombined plating build-up process is classified into a process using acopper foil with a resin, a process using a thermosetting resin, or aprocess using a photosensitive insulating resin, according to thematerial used and a method of forming a hole. Here, the method will bedescribed according to the process using a thermosetting resin in thecombined plating build-up process, which is often employed by thepresent inventors.

As shown in FIG. 8A, a core substrate 130 is prepared. This coresubstrate 130 is manufactured by the through-hole plating process. Aninner layer conductor pattern is formed on a glass-cloth epoxy-resincopper clad laminate or a glass-cloth high-heat-resistance-resin copperclad laminate. A desired number of these laminates are prepared, and arebonded with adhesive sheets known as prepreg to form a single sheet. Ahole is formed through the sheet, and a plated film 136 is formed on theinner wall and the surface of the sheet by means of the through-holeplating process so as to connect inner conductor layers and outerconductor layers. A surface pattern 134 is then formed, thus producingthe core substrate. Although this figure shows a multilayer coresubstrate that also includes a conductor circuit in the inner layer, thecore substrate may be a double-sided core substrate in which adouble-sided copper clad laminate is used as a starting material, noconductor circuit is provided in the inner layer, and the conductorcircuits provided on the top face and the bottom face are connected viaa through-hole conductor.

As shown in FIG. 8B, insulating layers 150 are formed both sides of thecore substrate 130, respectively. Each of the insulating layers 150 isformed by coating a liquid or by a laminating method in which a film isheated and bonded with pressure under a vacuum.

As shown in FIG. 8C, holes 150 a are formed on the surfaces of theinsulating layers 150, respectively.

As shown in FIG. 8D, electroless copper plating and electrolytic copperplating are performed on the inner surface of the hole 150 a and on thesurface of the insulating layer 150 so as to be electrically connectedwith each other. In this step, in order to increase the adhesiveness ofthe plating, a treatment for roughening a surface is performed on theinner surface of the hole 150 a and the surface of the insulating layer150.

As shown in FIG. 8E, a conductor pattern 158 is formed on the uppersurface as follows. Electrolytic copper plating is performed on theentire surface, that is, panel plating is performed to form a copperfilm 160. An etching resist is formed on the upper surface of the copperfilm. Subsequently, the conductor pattern 158 is formed by etching(subtractive process). Other methods such as the semi-additive processand the full-additive process may also be used.

As shown in FIG. 8F, similarly, a conductor pattern 158 is formed on thelower surface. At this step, a layer of the conductor pattern is formedon each side of the core substrate. Then, the steps shown in FIG. 8B toFIG. 8F are repeated a desired number of times.

As shown in FIG. 8G, in this embodiment, the steps shown in FIG. 8B toFIG. 8F are repeated once to manufacture a multilayer printed wiringboard. A solder resist layer (not shown) may be formed as an outermostlayers according to need. Although it is not apparent from FIGS. 8A to8C; conductor patterns 258 of the outermost layers are formed so as tomatch the patterns of the package 2 or the motherboard 4 shown inFIG. 1. An inductor is embedded in the completed package 2 by any one ofthe methods described in FIGS. 7A to 7C. The core substrate 130 shown inFIG. 8 or a printed wiring board prepared by alternately laminating aninterlayer resin insulating layer and a conductor circuit on a coresubstrate corresponds to the substrate 13 shown in FIGS. 7A to 7C. Inthe step shown in FIG. 8C, a through-hole is formed so as to penetratethe insulating layer 150, the core substrate 130, and the insulatinglayer 150, and an inductor is embedded in the through-hole by any methodshown in FIG. 7A to 7C. Thus, the inductor can be installed in the coresubstrate and the interlayer resin insulating layers.

[Advantages and Effects]

The above embodiments have the following advantages and effects.

(Inductor)

(1) An inductor having a low DC resistance can be provided. The inductorincludes a metal having a low resistivity, such as metallic copper, andhas a length (about 1 mm) smaller than the thickness of a board.Accordingly, the DC resistance of the inductor is very low.

(2) An inductor having a high inductance can be provided. The inductorhas a structure in which a conductor is in contact with a magnetic bodywith no gap therebetween, thereby increasing the inductance.

(3) An inductor having a small size can be provided. For example, theinductor has a radius of about 0.25 to 2 mm, and a length of about 1 mm.Thus, an inductor having a small size can be provided.

(4) An inductor that can be produced by the same manufacturing processas that of a printed circuit board can be provided. The process forproducing the inductor includes forming a hole and plating. Thus, theinductor can be produced by the same manufacturing process as that of aprinted wiring board.

(5) Because of its high inductance, the inductor can be used as a partof a power supply circuit through which a current of 0.1 A or more,preferably 1 A or more, flows.

(Inductor Embedded in Board)

(1) An inductor embedded in a board, which inductor requiring a mountingarea on a printed circuit board that is considerably smaller than themounting area of a surface mount device, can be provided. Since thisinductor is embedded in the board, another surface mount device can bemounted over the inductor. Thus, the mounting area of the inductor issubstantially equal to zero.

(2) The inductor can be disposed in the vicinity of a load. Since thisinductor is embedded in the board, the inductor can be located even inan area where the load is mounted. For example, the inductor can beembedded in a position of a printed wiring board or a core substratethat is disposed directly under an IC.

(3) An inductor that can be embedded by the same manufacturing processas that of a printed wiring board can be provided. This inductor can bemade in a printed wiring board by forming a hole, inserting theinductor, filling a resin, forming an opening, forming a via, andforming a conductor pattern. Thus, the inductor can be produced by thesame manufacturing process as that of the printed circuit board.

(Electronic Device)

(1) An electronic device including a power supply circuit disposed inthe vicinity of a load MPU can be provided. Since the distance from theoutput of the power supply circuit to the load MPU is very short, forexample, 1 mm or less, the length of the conductor circuit used for thepower supply is very short, and thus voltage variations due to aparasitic resistance or a parasitic inductance of wiring can besuppressed.

A specific example will now be described. FIG. 9 is a view showing anelectronic device 100 including a printed circuit board on which anexisting power supply circuit is mounted. The electronic device 100includes a package (PK) 200 and a motherboard (MB) 400 having thepackage 200 thereon. The package 200 and the motherboard 400 areelectrically connected to each other with, for example, pin junctions160.

A power supply IC 120, an inductor element 100, and a capacitor element80 that constitute the power supply circuit are surface mount type ofdiscrete devices. Each of these devices requires a certain mounting areaon the printed wiring board. Therefore, in this electronic device 100,these surface mount devices 120, 100, and 80 are mounted in the vicinityof an end of the power supply at an input voltage V_(in) of themotherboard (MB) 400. The power is supplied from the power supplycircuit to an MPU 60 through a conductor circuit 140 provided on themotherboard (MB) 400, the pin junctions 160, and conductor circuits(including a through-hole conductor and a via-hole conductor) 180 and280 etc. that are provided on the package 200. In this electronic device100, the distance from the output of the power supply circuit to the MPU60, which is a load, is very long, for example, in the range of severalcentimeters to 10 cm. Since the length of the conductor circuit used forthe power supply is very long, voltage variations due to a parasiticresistance or a parasitic inductance of wiring easily occur.

As compared with the known electronic device 100 shown in FIG. 9, in theelectronic device 1 according to the embodiment shown in FIG. 1, it isapparent that the power supply circuit and the load MPU 6 are very closeto each other.

[Modifications Etc.]

The embodiments of the inductor embedded in a board of the presentinvention and a power supply circuit including the inductor have beendescribed. These embodiments are illustrative and do not limit thepresent invention. Accordingly, additions, variations, modifications,and the like of these embodiments that may easily occur to those skilledin the art should be considered within the scope of the presentinvention. The scope of the present invention is defined on the basis ofthe description of the appended claims.

1. An inductor configured to be used as a part of a power supplycircuit, the inductor comprising: a cylindrical shaped magnetic bodyhaving opposing first and second end surfaces and a substantiallycylindrical shaped outer sidewall surface extending from the first endsurface to the second end surface; a through-hole extending from thefirst end surface to the second end surface of the cylindrical shapedmagnetic body and having a through-hole sidewall surface; and aconductor provided on the through-hole sidewall surface.
 2. The inductoraccording to claim 1, wherein the conductor comprises copper.
 3. Theinductor according to claim 1, wherein the conductor comprises copperand has a substantially cylindrical shape which is concentric with theouter sidewall surface of the magnetic body.
 4. The inductor accordingto claim 1, wherein the conductor comprises copper and has asubstantially hollow cylindrical shape.
 5. The inductor according toclaim 4, wherein a hollow portion of the conductor is substantiallycylindrically shaped and concentric with the conductor and the outersidewall of the magnetic body.
 6. The inductor according to claim 1,wherein the magnetic body comprises ferrite.
 7. The inductor accordingto claim 1, wherein the magnetic body comprises a composite materialcontaining a magnetic material and a nonmagnetic material.
 8. Theinductor according to claim 1, wherein the magnetic body comprises acomposite material containing a magnetic powder and a resin.
 9. Theinductor according to claim 1, wherein the magnetic body comprises acomposite material containing a carbonyl iron powder and a resin. 10.The inductor according to claim 1, further comprising a dielectricmaterial, which substantially surrounds the cylindrical magnetic body.11. An inductor configured to be inserted in a through-hole of amultilayer printed circuit board such that the inductor is embedded inthe multilayer printed circuit board, the inductor comprising: amagnetic body having opposing first and second end surfaces and an outersidewall surface extending from the first end surface to the second endsurface, the magnetic body having a predetermined shape based on a shapeof a through hole in which the magnetic body will be inserted; amagnetic body through-hole extending from the first end surface to thesecond end surface of the magnetic body and having a through-holesidewall surface; and a conductor provided on the through-hole sidewallsurface, the conductor forming a longest coil dimension of the inductor.